An aircraft usually follows a predetermined flight plan comprising a detailed description of the predetermined path to follow in the context of a previously planned flight. It notably comprises a path, which is a chronological sequence of waypoints described by their position, altitude and passage time. These paths are allocated with air corridors extending around this route. The size of the air corridors allocated to the aircraft depends on the flight phase (takeoff, cruise, approach, landing). Given the saturation of airspace, there is a tendency to reduce the size of the corridors in order to open a larger number of corridors in airspace.
There are procedures of the RNP (Required Navigation Performance) type whose purpose is to reduce the size of the air corridor allocated to an aircraft, to reduce the spacing between aircraft and the clearance of obstacle margins (distances). The big advantage of RNP procedures is that they allow an aircraft to fly to and from places of difficult access with good guarantees in terms of safety. These procedures also make it possible to carry out curved approaches and to descend lower during approach phases in order to eventually detect, at the end of the approach phase, the landing system or in order that the pilot can decide, according to the visibility, if he can make the landing.
In these RNP procedures, an air volume is allocated to the aircraft for each phase of the flight. In order to maneuver inside the airspaces defined in an RNP procedure, the navigation system of the aircraft must be capable of monitoring its performance and of comparing it with an alarm limit defined for the approach phase. The navigation system must also be capable of informing the crew or the automatic pilot system if its performance reaches the alarm limit. This makes it possible for the instruction giver to guarantee a safety level over the approach phase or to be aware that the safety level is not guaranteed.
One of the key performance factors of a navigation system is its integrity. The integrity is the ability of a system to give a warning when its performance degrades beyond a predefined threshold.
In order to quantify the integrity of a position measurement in aeronautical applications, where the integrity is critical, a parameter called the “protection radius” of the position measurement as well as an associated detection probability (that is to say a given level of integrity) are used. By definition, the probability that the position error exceeds the announced protection radius without an alarm being sent is less than this probability value. For example, in FIG. 1 there is shown a protection radius R for an RNP 0.1 procedure. In order to follow an RNP 0.1 procedure, it is necessary that the probability that the path error E, in the horizontal direction, exceeds 0.2 nautical miles (i.e. 0.2N), without an alarm being sent to the instruction giver, should be less than 10−5 per hour. The position error E is the distance between the true position Ptrue and the desired theoretical position Pth. An alarm limit Rlim is therefore defined for this procedure which is equal to 0.2 Nautical Miles. If a protection radius R(t) calculated by the navigation system, for the probability of appearance of error less than or equal to 10−5 per hour, exceeds this alarm limit Rlim, the instruction giver is informed of it. It will be noted that one nautical mile is equal to 1,852 meters.
To this end, in RNP procedures, the navigation systems usually calculate continuously the position of the aircraft and the value of the horizontal and/or vertical protection radii associated with this position, for a given integrity level.
In order to follow RNP procedures, the aircraft can be equipped with navigation systems of the INS/GNSS (Inertial Navigation System and Global Navigation Satellite System) type. INS/GNSS navigation systems comprise at least one inertial system (inertial sensors and associated calculator) and a receiver of satellite information. These systems are based on satellite information and/or inertial information in order to determine the successive positions of the aircraft. The information coming from satellites makes it possible to provide accurate position measurements with a good level of integrity (small protection radius or a low error probability). On the other hand, the satellite navigation information is subject to being lost. It is said that the satellite navigation information is lost when the satellite navigation information is insufficient for the navigation system to calculate the position of the aircraft from the information coming from the visible satellites. This is the case for example when the receiver of position information from the satellites is defective or when certain satellites are hidden by obstacles and the receiver sees less than four satellites or when the receiver sees more than four satellites but they are in a configuration which does not make it possible to calculate the position of the aircraft (for example when all of the visible satellites are aligned).
As for the information coming from the inertial systems, this makes it possible to provide position measurements continuously but these measurements drift in the long term. Generally, the navigation system calculates the position of the aircraft on the basis of satellite information and, when the satellite navigation information is lost, the position calculations are carried out on the basis of inertial information.
A navigation system which makes it possible to follow RNP procedures is known from the patent application WO2008/040658. The navigation system is a hybrid system of the closed loop INS/GNSS hybrid type. The hybridizing consists in mathematically combining the position and speed information provided by the inertial system and the measurements provided by a satellite positioning receiver in order to obtain the position measurements by taking the advantages of both systems, that is to say the continuity of the information provided by an inertial receiver and the accuracy of the information provided by the satellites. This navigation system continuously corrects the drift of the inertial information by basing itself on the information coming from the satellites. This navigation system is capable of continuously monitoring its performance by calculating horizontal and vertical protection radii for a given level of integrity and by comparing these protection radii with an alarm limit defined over the current flight phase.
Over each approach phase is defined a Decision Altitude (DA), defined with respect to the mean sea level, or a Decision Height (DH), defined relatively with respect to the destination runway threshold, which is the altitude or height at which the aircraft is at the end of the approach procedure and at which the pilot will decided whether or not to commence the landing phase. This height depends on the type of approach (non-precision approach, precision approach) and on the chosen means of approach (visual or instrument). For each airport, a Minimum Safety Altitude (MSA) is defined which is the altitude which the aircraft must be at to be sure of not striking a relief, that is to say in order to guarantee its safety. The safety altitude is conditioned by the relief situated in the vicinity of the runway upon which the landing is planned. At present, when an aircraft which is following an RNP approach phase loses the satellite navigation information the instruction giver is informed of this and then decides either to continue the current flight phase, but without guaranteeing the safety level imposed over the approach phase, or to stop the current flight phase by simultaneously extracting himself from the predetermined path in order to reach the safety altitude.
If, for example, on a predetermined path, shown in thick continuous line in FIG. 2, followed during an approach phase using the RPN procedure, an aircraft comprising a navigation system based on satellite and inertial information loses the satellite navigation information at a loss time tp, the instruction giver is warned of the loss of information and he instantaneously interrupts his initial path in order to reach a safety altitude hsafe by following a path called the “extraction path”, shown in thick dotted line. When the aircraft has reached the safety altitude, he will either attempt a new approach procedure or abandon the current approach in order to follow a third path.
This type of procedure has the disadvantage of interrupting the current approach when the satellite navigation information is no longer available, even if the navigation system would finally have been able to guarantee, due to the hybridizing, the safety conditions required over the approach phase up to the end of this phase. Moreover, if the current approach is not interrupted, the safety conditions are no longer guaranteed. This lengthens the duration of flights and results in additional costs in terms of fuel, crew remuneration and saturation of airspace.